1. Technical Field
The present invention relates to a black ink composition suitable for an inkjet recording method wherein not only is print quality enhanced and image fastness of a printed product guaranteed, but also bronzing is suppressed, fastness of a recorded matter (printed product) is excellent, and a beautiful black color can be sustained for a long period of time; an ink set comprising the aforementioned black ink composition; an ink cartridge containing the aforementioned black ink composition; and an inkjet recording method and recorded matter using the same.
In addition, the present invention relates to a black ink composition that approaches achromicity under medium duty conditions wherein the aforementioned properties are retained and the hue of the printed product is guaranteed by mixing therewith a specific complementary coloring material; an ink set comprising the aforementioned black ink composition; an ink cartridge containing the aforementioned black ink composition; and an inkjet recording method and recorded matter using the same.
2. Related Art
The inkjet method is a method wherein ink droplets are ejected from a fine nozzle head, and letters and graphics are recorded on the surface of a recording medium such as paper and the like. Inkjet recording methods that have reached practical application include a method wherein electric signals are converted to mechanical signals using piezoelectric elements, ink droplets stored in the nozzle head member are ejected intermittently therefrom, and letters and symbols are recorded on the surface of the recording medium; and a method wherein portions of ink liquid are rapidly heated at members near the nozzle head ejecting member to generate bubbles, ink droplets are ejected intermittently by the expansions in volume caused thereby, and letters and symbols are recorded on the surface of the recording medium.
Inks used in inkjet recording are generally those wherein based on their safety and printing properties, various colorants are dissolved in water, an organic solvent, or a mixture thereof, but more stringent conditions are required than for the ink compositions used in writing instruments such as fountain pens and ball point pens.
For example, when high duty printing such as solid printing (printing with a 100% duty fill) is performed on a substrate such as gloss paper and the like, places that should appear as ordinary white light can appear to be colored when observed under specular reflection conditions, and differences between the recording medium itself (non-inked parts) and the reflected light arise, giving the impression that the printed part is floating. This is due to coagulation of the colorants that is generally called bronzing, and is a factor that makes the printed letters and figures difficult to see. This phenomenon is often conspicuous with strongly weather-resistant colorants, and improvement is needed.
Especially in the case of black ink, bronzing causes a change in hue and decrease in coloring that result in the loss of image sharpness, a problem that strongly needs to be resolved.
This problematic bronzing phenomenon is caused by the crystallization of the dye when the ink composition dries on the surface of the paper. Therefore, the following methods have generally been used to correct bronzing.
The first method is one wherein an alcohol-amine is added to the ink composition as a pH stabilizer. This method corrects bronzing by improving the solubility of the dye.
The second method increases the penetration of the ink composition into the paper. This method corrects bronzing by causing the ink composition to penetrate into the paper before the dye crystallizes.
However, the first method has the following problems: (1) the alcohol amine raises the pH of the ink composition to near 11, and when this ink composition is used in an inkjet printer, corrosion of the printer nozzles occurs, and (2) because modern highly weather-resistant colorants have poor storage stability in an alkali environment, when a highly weather-resistant colorant is used as the coloring agent, the storage stability of the ink composition deteriorates.
Moreover, the second method has the following problems: (1) the penetration of the ink composition changes depending on the type and properties of the paper, and (2) when the penetration of the ink into the paper is excessive, so-called bleeding occurs as well as other decreases in printing quality.
Therefore, the correction of bronzing by adding an amino acid to the ink composition has been proposed in place of adding a pH stabilizer or increasing the penetration of the ink composition (JP-A-6-25575 and JP-A-7-228810).
However, the effectiveness in inhibiting bronzing by the addition of an amino acid was insufficient when a highly weather-resistant colorant was used as the coloring agent.